Manipulating fluidic reagents and assessing the results of reagent interactions are central to chemical and biological science. Manipulations include mixing fluidic reagents, assaying products resulting from such mixtures, and separation or purification of products or reagents and the like. Assessing the results of reagent interactions can include autoradiography, spectroscopy, microscopy, photography, mass spectrometry, nuclear magnetic resonance and many other techniques for observing and recording the results of mixing reagents. A single experiment may involve literally hundreds of fluidic manipulations, product separations, result recording processes and data compilation and integration steps. Fluidic manipulations are performed using a wide variety of laboratory equipment, including various fluid heating devices, fluidic mixing devices, centrifugation equipment, molecule purification apparatus, chromatographic machinery, gel electrophoretic equipment and the like. The effects of mixing fluidic reagents are typically assessed by additional equipment relating to detection, visualization or recording of an event to be assayed, such as spectrophotometers, autoradiographic equipment, microscopes, gel scanners, computers and the like.
Because analysis of even simple chemical, biochemical, or biological phenomena requires many different types of laboratory equipment, the modern laboratory is complex, large and expensive. In addition, because so many different types of equipment are used in even conceptually simple experiments such as DNA synthesis or sequencing, it has not generally been practical to integrate different types of equipment to improve automation. The need for a laboratory worker to physically perform many aspects of laboratory science imposes sharp limits on the number of experiments which a laboratory can perform, and increases the undesirable exposure of laboratory workers to toxic or radioactive reagents.
One particularly labor intensive biochemical series of laboratory fluidic manipulations is nucleic acid synthesis and analysis. A variety of in vitro amplification methods for biochemical synthesis of nucleic acids are available, such as the polymerase chain reaction (PCR). See, Mullis et al., (1987) U.S. Pat. No. 4,683,202 and PCR Protocols A Guide to Methods and Applications (Innis et al. eds, Academic Press Inc. San Diego, Calif. (1990) (Innis). PCR methods typically require the use of specialized machinery for performing thermocycling reactions to perform DNA synthesis, followed by the use of specialized machinery for electrophoretic analysis of synthesized DNA. For a description of nucleic acid manipulation methods and apparatus see Sambrook et al. (1989) Molecular Cloning—A Laboratory Manual (2nd ed.) Vol. 1–3, Cold Spring Harbor Laboratory, Cold Spring Harbor Press, NY, (Sambrook); and Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., Current Protocols, a joint venture between Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., (1997, supplement 37) (Ausubel).
Another particularly important and labor intensive biochemical series of laboratory fluidic manipulations which are typically performed on nucleic acids which are made recombinantly or synthetically is nucleic acid sequencing. Efficient DNA sequencing technology is central to the development of the biotechnology industry and basic biological research. Improvements in the efficiency and speed of DNA sequencing are needed to keep pace with the demands for DNA sequence information. The Human Genome Project, for example, has set a goal of dramatically increasing the efficiency, cost-effectiveness and throughput of DNA sequencing techniques. See, e.g., Collins, and Galas (1993) Science 262:43–46.
Most DNA sequencing today is carried out by chain termination methods of DNA sequencing. The most popular chain termination methods of DNA sequencing are variants of the dideoxynucleotide mediated chain termination method of Sanger. See, Sanger et al. (1977) Proc. Nat. Acad. Sci., USA 74:5463–5467. For a simple introduction to dideoxy sequencing, see, Ausubel or Sambrook, supra. Four color sequencing is described in U.S. Pat. No. 5,171,534. Thousands of laboratories employ dideoxynucleotide chain termination techniques. Commercial kits containing the reagents most typically used for these methods of DNA sequencing are available and widely used.
In addition to the Sanger methods of chain termination, new PCR exonuclease digestion methods have also been proposed for DNA sequencing. Direct sequencing of PCR generated amplicons by selectively incorporating boronated nuclease resistant nucleotides into the amplicons during PCR and digestion of the amplicons with a nuclease to produce sized template fragments has been proposed (Porter et al. (1997) Nucleic Acids Research 25(8):1611–1617). In the methods, 4 PCR reactions on a template are performed, in each of which one of the nucleotide triphosphates in the PCR reaction mixture is partially substituted with a 2′deoxynucleoside 5′-α[P-borano]-triphosphate. The boronated nucleotide is stocastically incorporated into PCR products at varying positions along the PCR amplicon in a nested set of PCR fragments of the template. An exonuclease which is blocked by incorporated boronated nucleotides is used to cleave the PCR amplicons. The cleaved amplicons are then separated by size using polyacrylamide gel electrophoresis, providing the sequence of the amplicon. An advantage of this method is that it requires fewer biochemical manipulations than performing standard Sanger-style sequencing of PCR amplicons.
Other sequencing methods which reduce the number of steps necessary for template preparation and primer selection have been developed. One proposed variation on sequencing technology involves the use of modular primers for use in PCR and DNA sequencing. For example, Ulanovsky and co-workers have described the mechanism of the modular primer effect (Beskin et al. (1995) Nucleic Acids Research 23(15):2881–2885) in which short primers of 5–6 nucleotides can specifically prime a template-dependent polymerase enzyme for template dependent nucleic acid synthesis. A modified version of the use of the modular primer strategy, in which small nucleotide primers are specifically elongated for use in PCR to amplify and sequence template nucleic acids has also been described. The procedure is referred to as DNA sequencing using differential extension with nucleotide subsets (DENS). See, Raja et al. (1997) Nucleic Acids Research 25(4):800–805.
Improvements in methods for generating sequencing templates have also been developed. DNA sequencing typically involves three steps: i) making suitable templates for the regions to be sequenced (i.e., by synthesizing or cloning the nucleic acid to be sequenced); ii) running sequencing reactions for electrophoresis, and iii) assessing the results of the reaction. The latter steps are sometimes automated by use of large and very expensive workstations and autosequencers. The first step often requires careful experimental design and laborious DNA manipulation such as the construction of nested deletion mutants. See, Griffin, H. G. and Griffin, A. M. (1993) DNA sequencing protocols, Humana Press, New Jersey. Alternatively, random “shot-gun” sequencing methods, are sometimes used to make templates, in which randomly selected sub clones, which may or may not have overlapping sequence information, are randomly sequenced. The sequences of the sub clones are compiled to produce an ordered sequence. This procedure eliminates complicated DNA manipulations; however, the method is inherently inefficient because many recombinant clones must be sequenced due to the random nature of the procedure. Because of the labor intensive nature of sequencing, the repetitive sequencing of many individual clones dramatically reduces the throughput of these sequencing systems.
Recently, Hagiwara and Curtis (1996) Nucleic Acids Research 24(12):2460–2461 developed a “long distance sequencer” PCR protocol for generating overlapping nucleic acids from very large clones to facilitate sequencing, and methods of amplifying and tagging the overlapping nucleic acids into suitable sequencing templates. The methods can be used in conjunction with shotgun sequencing techniques to improve the efficiency of shotgun methods.
In an attempt to increase laboratory throughput and to decrease exposure of laboratory workers to reagents, various strategies have been performed. For example, robotic introduction of fluids onto microtiter plates is commonly performed to speed mixing of reagents and to enhance experimental throughput. More recently, microscale devices for high throughput mixing and assaying of small fluid volumes have been developed. For example, U.S. Pat. No. 6,046,056 provides pioneering technology related to microscale fluidic devices, especially including electrokinetic devices. The devices are generally suitable for assays relating to the interaction of biological and chemical species, including enzymes and substrates, ligands and ligand binders, receptors and ligands, antibodies and antibody ligands, as well as many other assays. Because the devices provide the ability to mix fluidic reagents and assay mixing results in a single continuous process, and because minute amounts of reagents can be assayed, these microscale devices represent a fundamental advance for laboratory science. Pioneering integrated systems for nucleic acid sequencing utilizing microfluidic fluid manipulation are described in, e.g., provisional patent application U.S. Ser. No. 60/068,311, entitled “Closed Loop Biochemical Analyzer” by Knapp, filed Dec. 19, 1997 and U.S. Pat. No. 6,235,471.
In the electrokinetic microscale devices and systems provided by Parce et al. and Knapp above, an appropriate fluid is flowed into a microchannel etched in a substrate having functional groups present at the surface. The groups ionize when the surface is contacted with an aqueous solution. For example, where the surface of the channel includes hydroxyl functional groups at the surface, e.g., as in glass substrates, protons can leave the surface of the channel and enter the fluid. Under such conditions, the surface possesses a net negative charge, whereas the fluid will possess an excess of protons, or positive charge, particularly localized near the interface between the channel surface and the fluid. By applying an electric field along the length of the channel, cations will flow toward the negative electrode. Movement of the sheath of positively charged species in the fluid pulls the solvent with them.
Although improvements in robotic manipulation of fluidic reagents and miniaturization of laboratory equipment have been made, and although particular biochemical processes such as DNA amplification and sequencing are very well developed, there still exists a need for additional techniques and apparatus for mixing and assaying fluidic reagents, for integration of such systems and for reduction of the number of manipulations required to perform biochemical manipulations such as DNA sequencing. Ideally, these new apparatus would be useful with, and compatible to, established biochemical protocols. This invention provides these and many other features.